1. Field of the Invention
The present invention relates to a developing roller, a process cartridge, and an electrophotographic apparatus.
2. Description of the Related Art
A nonmagnetic, one-component, contact developing mode has been currently attracting attention as an electrophotographic image-forming method. In the mode, toner is supplied onto the surface of a developing roller by a toner-supplying roller provided to abut the developing roller. Next, excessive toner on the surface of the developing roller is removed by a toner-regulating member so that a toner layer may be formed in a thin-film fashion on the developing roller. At the same time, toner particles are provided with a predetermined quantity of positive or negative triboelectric charge by rubbing. Further, the toner subjected to the positive or negative triboelectric charging is conveyed through the rotation of the developing roller, and then the toner is caused to adhere to an electrostatic image on the surface of an electrophotographic photosensitive member (also referred to as “photosensitive member”) placed to contact the developing roller so that the image may be developed. Such developing roller generally has such a construction that an elastic layer is provided for the periphery of a conductive mandrel and a surface layer is provided for its outer periphery as required.
By the way, an electrophotographic apparatus has been requested to stably provide high-quality electrophotographic images under a wide variety of environments. However, it is difficult for the apparatus to stably output high-quality electrophotographic images under each of a high-temperature, high-humidity environment (having, for example, a temperature of 40° C. and a humidity of 95% RH) and a low-temperature, low-humidity environment (having, for example, a temperature of 0° C. and a humidity of 10% RH).
Specifically, when a developing roller is mounted on a process cartridge or an electrophotographic apparatus, the developing roller may be stored under a high-temperature, high-humidity environment over a long period of time. In such case, a small amount of an unreacted product present in the elastic layer or surface layer of the developing roller has sometimes deposited (bled) onto the outermost surface of the developing roller. This is probably because of the following reason. Under high temperature and high humidity, the molecular mobility of a polymer for forming the elastic layer or surface layer of the developing roller is raised so that the unreacted product may be apt to migrate toward the surface. When the developing roller to the surface of which such unreacted product has bled is used in the formation of an electrophotographic image, unevenness may occur in the electrophotographic image.
On the other hand, under a low-temperature, low-humidity environment, a stress to be applied to toner may become excessively strong owing to a relative increase in the surface hardness of the developing roller. As a result, the toner has fused to the surface of the developing roller in some cases.
Japanese Patent Application Laid-Open No. H09-12192 describes a method involving using an ester polyol derived from 2,4-diethyl-1,5-pentanediol as a urethane raw material for the purpose of suppressing the environment dependence of a rubber member for electrophotography.
In addition, Japanese Patent Application Laid-Open No. H11-212354 discloses, for the purpose of suppressing toner fusion (melt-adhesion) under a low-temperature, low-humidity environment, a construction having a mandrel and a urethane elastic layer on its outer periphery, and having, on the peripheral surface of the layer, a surface layer containing a polyurethane having a polysiloxane skeleton in a molecule thereof.